Soft white rock (5)
I believe the answer is:
chalk
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">soft white rock</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for chalk that I've seen before include "Soft white rock used for writing on boards etc" , "Blackboard writing implement" , "Substance totally different from cheese!" , "Calcium carbonate" , "Opposite of cheese?" .)